1. Field of the Invention
The present invention relates to a light guiding panel assembly used for a display or a signboard in which a positive film disposed on a surface of a guiding plate is illuminated from backward by a guided light from a light source via the guiding plate, and used for a surface illumination panel with a guided light via the guiding plate.
2. Description of the Related Art
Patent document 1 listed below by an inventor of this application discloses such a light guide panel assembly. According to this document, the light guide panel assembly includes: a light guiding plate made of synthetic resin, and guiding light provided via an incident end face from a light source; a base plate disposed on a rear wall of the guiding plate; and the light source such as an LED unit disposed adjacent to the incident end face of the guiding plate. The light guide panel assembly further includes: a light diffuser plate disposed on a front wall of the guiding plate; a transparent cover disposed on a front wall of the light diffuser plate; and a frame covering each end of these components, and integrally fixing the base plate with a fixing means such as adhesive, screw or the like. Thus, a positive film is clipped between the light diffuser plate and the transparent cover, and used as a display or a signboard by back-lighting of the positive film with a guided light via the light guiding plate. In this case, the light source is an LED unit composed of LED chips arranged series on a substrate. When the substrate is fixed to an inner bottom wall of the frame with the fixing means such as adhesive, the incident light from the LED chips of the LED unit is supplied to the light guiding plate.    [Patent Document 1] JP, A, 2009-123557
In this case, a thin light guiding panel assembly which is relatively large-sized such as A1 size, B1 size, or a large panel 1 to 2 meters on a side can illuminate back-lighting with high brightness. However, because the light guiding plate is made of synthetic resin such as an acrylic transparent resin, when the light guiding panel assembly is located at a high-temperature place, for example, the light guiding panel assembly exposed outdoors facing south and receiving the direct light of the sun, due to heat expansion of the guiding plate, there is a possibility that the light guiding panel assembly may not light and the back-lighting may be impossible.
In other words, when the light source is the LED unit, the LED unit is fixed to the inner bottom wall of the frame. A heat expansion coefficient of the acrylic transparent resin plate is 6.8×10−5/K. When the temperature is changed in 10 degree C., the plate is expanded or contracted 0.7 mm per 1 meter in both width and height direction. Thus, the heat expansion or heat contraction of the light guiding plate is slightly over 2 mm each at the temperature change of 30 degree C., slightly less than 3 mm each at the temperature change of 40 degree C., and slightly less than 3.5 mm at the temperature change of 50 degree C. Therefore, when the frame having the light source is fixed to the base plate, for example, due to the heat expansion at the incident end face in a width direction of the light guiding plate, the incident end face of the light guiding plate pushes the light source, and due to the heat expansion in a height direction of the light guiding plate, a tensile force is applied to the LED unit to expand the LED unit. Resultingly, a substrate of the LED unit or a connecting part of the LED unit may be broken, or damaged.
In this case, when a gap is provided between the light guiding plate and the light source, the gap absorbs the heat expansion of the light guiding plate to prevent the LED unit from being broken or damaged. However, when a gap is provided, incident light of the light source is decreased inversely proportional to the square of a distance of the gap, and the illumination brightness of the light guiding panel assembly is decreased. The gap which generates no decrease of the illumination brightness is limited to about 1 mm. Therefore, even if such a gap is provided, when the thermal expansion exceeds the gap, similarly, the tensile force is applied to the LED unit and the LED unit may be broken or damaged.
Recently, a large-sized light guiding panel assembly has been installed out of doors such as a time table on a station platform, a guiding board in an open space in front of a building or on a wall surface of a building. Therefore, it is necessary to eliminate the possibility of a trouble caused by heat expansion or contraction of the light guiding plate accompanying the temperature change.
Accordingly, an object of the present invention is to provide a light guiding panel assembly used for a display, a signboard, a surface illumination or the like configured to simply and surely prevent a trouble caused by heat expansion or contraction without causing the decrease of illumination brightness while using a synthetic resin made light guiding plate in which the heat expansion or contraction is inevitable.